Heat the vegetable oil in a deep-fat fryer to 160C/320F. (CAUTION: Hot oil can be dangerous. Do not leave unattended.)
Mix together all of the dry ingredients in a bowl. In a separate bowl, beat the egg and milk together and gradually stir the mixture into to the dry ingredients. Whisk together to make a heavy batter.
Put a skewer through each frankfurter sausage and dip it into the batter until well coated.
Place the frankfurters into the deep-fat fryer, a few at a time, and cook for three minutes, or until golden-brown. Drain on a plate lined with kitchen paper.
To serve, mix the mustard and mayonnaise together in a bowl and serve with the frankfurters.